Have you tried to go to the net pop to disconnect the line. Are you sure it's on the building side of the netpop?

I had a problem with the lines in the back alley of the building I moved into. I guess there are multiple taps on the terminals on the poles and Bell here has a habit of not removing old wire from the terminals down the alley.

When I got hooked up, some lady down the street started using my phone line because her phone rang. She gave her friends my phone number and she answered the calls when they rang in on her phone. I didn't catch it right away because it was on my fax line.

The apartment I live in has a 50 pair cable coming in from one side of the building and out to the other. There are about 10 buildings this cable goes through. They dropped an aerial to my building and it solved the problem.

Tip and ring, the work of the devil!